Hello,
I'm going on a yearlong expedition to Ecuador where I will be photographing and recording video in the Amazon jungle and Andes Mountains with my first interchangeable lens camera (haven't decided which, leaning towards the OMD-E5, A77, 60D, or GH3; that will be a different post). I currently have an old trusty Macbook Pro from late 2006, a painfully sluggish (but unlocked) iPhone 3G 16GB, and before knowing I was going on the expedition, an iPad 3rd Gen. 64GB Verizon (wish I waited on that one, but here I am).

I'm now considering whether this set up will work in Ecuador or if I should replace something while I still can. Particularly, I'm wondering if the iPad will be more useful to me there than say an unlocked iPhone 5? Or replace the iPad with a new Macbook Pro instead of upgrading when I get back (conditions there will be incredibly humid, so I felt more comfortable risking my old mac with the intention of replacing it when I get back, also my partner has a new Macbook Pro that I can use from time to time while there).

Replace your Mac after getting back; a proper computer is infinitely more useful than a tablet can ever be.
As for the usefulness of your iPad, it is probably going to cost you a pretty penny for roaming data but if you don't want that it won't be very useful.
A Verizon iPhone 5 will only be locked on the LTE network; it is already unlocked for GSM use! Which means you can get an on-contract iPhone 5 for $200-400 and use a GSM SIM card for Ecuador. It's twice as fast and works better than the iPad. Also more pocketable too!

So what I'd recommend is first replace your iPhone 3G with an iPhone 5 on Verizon, and replace your Mac and iPad with a brand spankin' new Mac.